It is desired to have a light weight bearing apparatus that is compact and highly durable that provides fuel consumption efficiency and performance for automobiles. There are driven wheel types and drive wheel types of automobile wheel bearing apparatus. Weight reduction of the bearing apparatus for a driven wheel has been achieved by reducing the thickness of the wheel hub wheel mounting flange and by forming a through bore. In addition, a driving wheel bearing apparatus having a wheel hub with a reduced thickness wheel mounting flange and a serrated through bore is generally used. In the driven wheel bearing apparatus, a stationary ring is adapted to be secured on a body of the vehicle and a rotational ring is connected to support and rotate a wheel of the vehicle. On the other hand, in the driving wheel bearing apparatus, a stationary ring is adapted to be secured on a body of the vehicle and a rotational ring is connected to support and rotate a wheel of the vehicle while transmitting the engine power to the driving wheel.
There are four generation types of wheel bearing apparatus. A first generation type has a wheel bearing including a double row angular contact ball bearing fit between a knuckle, forming a portion of a suspension apparatus, and a wheel hub. A second generation type has a body mounting flange or a wheel mounting flange directly formed on the outer circumference of an outer member. A third generation type has one inner raceway surface directly formed on the outer circumference of a wheel hub. A fourth generation type has inner raceway surfaces formed on the outer circumferences, respectively, of a wheel hub and an outer joint member.
In the wheel bearing apparatus of the third generation type, a wheel hub is integrally formed with a wheel mounting flange. An inner raceway surface is formed directly on an outer circumference of a shaft portion extending from the base of the wheel mounting flange. The wheel hub generally has several machining steps performed on it, e.g., removing surface scale by shot blasting after being formed by forging, turning several functional portions such as the inner raceway surface by a dedicated turning line, and transferring to a high frequency induction quenching step and a grinding step. The wheel hub is usually made of steel such as S53C. Portions of the wheel hub, such as the inner raceway surface, are finally hardened by a high frequency quenching.
In such a wheel bearing apparatus of a third generation type, one problem is its durability as a structure for rotationally supporting the wheel hub. Methods to improve the durability of the wheel hub have been proposed. For example, technology providing hardened layers on an outer circumference of a shaft portion of a wheel hub and on a root region of a wheel mounting flange and brake pilot has been disclosed (see Patent Document 1 below). Also, technology providing a hardened layer on an outer circumference (including an inner raceway surface) of a shaft portion of a wheel hub and thermal refining a non-hardened portion has been disclosed (see Patent Document 2 below). In addition, the wheel hub is required to have a raceway function of a rolling bearing. Thus, a wheel bearing apparatus where the inclination angle has been proposed where fiber flow in an inner raceway surface of a wheel hub is set at 15° or less. Also proposed is a reduction in the machining allowance of the inner raceway surface to reduce the amount of material forming the wheel hub and a time required for the cutting process (see Patent Document 3 below). Patent Document 1: Japanese Laid-open Patent Publication No. 87008/2002. Patent Document 2: Japanese Laid-open Patent Publication No. 3061/2005. Patent Document 3: Japanese Laid-open Patent Publication No. 83513/2005
An improvement in durability and the life of rolling fatigue of the wheel hub can be achieved by the technologies disclosed in the Patent Documents 1˜3 above. However, further problems exist in the manufacturing process of a wheel hub that will be described with reference to FIG. 9. A wheel hub 50 is finally finished to a configuration as shown by a two-dotted line and a shoulder portion 51. An inner ring (not shown) is abutted against the shoulder portion 51. A cylindrical portion 50b extends from the shoulder portion 51. Repeating moment loads applied to the wheel mounting flange 54 of the wheel hub 50 are transmitted to a shaft portion 56 that extends from the base of the wheel mounting flange 54. The loads repeatingly cause elastic deformation on the wheel hub 50. Thus, repeated bending stress is generated in the shoulder portion 51 of the wheel hub 50. To assure a desired strength of the wheel hub 50, it is hardened by high frequency induction quenching to form a hardened layer 58 on an outer circumference of the shaft portion 56 in a region from an inner side base corner 57 (seal land portion) of the wheel mounting flange 54 to the cylindrical portion 50b, via an inner raceway surface 50a and the shoulder portion 51.
It is a usual manner to provide a through bore in an inner circumference 59 of the shaft portion 56 of wheel hub 50 to reduce the weight of the wheel hub 50 or to form a serration for torque transmission in a driving wheel bearing apparatus. In either case, the wall thickness of the cylindrical portion 50b where the inner ring is fit is reduced by the provision of the trough bore. A necessary strength of the cylindrical portion 50b is assured by providing the hardened layer 58. However, it is possible that cracks may be caused on the inner circumference 59 corresponding to the shoulder portion 51 where maximum deformation of the shaft portion 56 is exposed to the generation of high repeating stress.
In fact, the wheel hub 50 is forged with portions to be finished late to form the inner raceway surface 50a on the outer circumference of the shaft portion 56 and to form the cylindrical portion 50b. Finally, the wheel hub 50 is punched to form an inner circumference 55 with a remaining partition wall 52 to be later punched out. In this step, since an outer side wall surface 53 of the partition wall 52 is formed more on the outer side than the shoulder portion 51, as shown in FIG. 10 (a), the fiber flow of the wheel hub 50 is formed to be inclined along a direction from the shoulder portion 51 to the partition wall. The partition wall 52 is removed by the punching-out step. The inner circumference 55 is formed as a through bore (see FIG. 10 (b)). Thus, the fiber flow in a portion opposite to the shoulder 51 exhibit a condition cut at an angle about 50°˜80°. In addition, a trimming step, to remove burrs (not shown) on the outer circumference of the wheel mounting flange, is performed directly before or directly after or simultaneously with the punching-out step.
In general, it is known that higher strength against rotating bending fatigue and impact bending can be obtained in a case where the fiber flow is parallel with the direction of the bending stress than a case where the fiber flow is vertical to the direction of the bending stress. Accordingly, it is difficult to have a predetermined strength in the inner circumference 55 where the fiber flow is not parallel relative to its axis. It may be appreciated to harden, by high frequency quenching, the through bore of the shaft portion 56 after the turning step to increase the strength. However, it not only causes an increase in manufacturing cost of the wheel bearing and in the generation of strain due to the heat treatment but reduces the impact resistance of the shaft portion 56 due to the quenching defect. Accordingly, the thickness of the cylindrical portion 50b must be increased in practice to obtain sufficient strength.